A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor, comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles (e.g., zinc oxide) and a resin binder, is subjected to an ordinary electrophotographic processing to form a highly lipophilic toner image thereon. The surface of the photoreceptor is then treated with an oil-desensitizing solution called etching solution to selectively render non-image areas hydrophilic to obtain an offset printing plate.
Requirements of offset printing plate precursors for obtaining satisfactory prints are such that an original should be reproduced faithfully on the photoreceptor; the surface of a photoreceptor should have affinity with an oil-desensitizing solution, so as to render non-image areas sufficiently hydrophilic and, at the same time, should have water resistance; and that a photoconductive layer having an image formed thereon is not released during printing and is well receptive to dampening water, so that the non-image areas hold the hydrophilic properties enough to be freed from stains even on printing a large number of prints.
For particular use as an offset printing plate precursor, formation of background stains due to insufficient oil-densensistivity presents a serious problem. In order to solve this problem, various resins as binders for zinc oxide have been proposed. For example, such resins have been disclosed in Japanese Patent Publication No. 31011/75, Japanese patent application (OPI) Nos. 54027/78, 20735/79, 202544/82, and 68046/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Nevertheless, actual evaluations of these resins proposed for improving oil-desensitivity have revealed that none were satisfactory in terms of stain resistance and printing durability.
On the other hand, a photoconductive layer comprising particles of an organic photoconductive compound and a binder resin can be formed on a grained aluminum base. Printing plate precursors of this type can be produced by forming a toner image on a photosensitive layer through the known electrophotographic processing such as described above and further removing the non-image areas by eluting with a processing solution, whereby the aluminum base corresponding to the non-image areas is exposed to form hydrophilic areas. The photosensitive layer using the organic photoconductive compound comprises, for example, an oxadiazole compound or oxazole compound and an alkali-soluble binder resin (e.g., a styrene-maleic anhydride copolymer, etc.), as disclosed in Japanese Patent Publication Nos. 17162/62 and 39405/71 and Japanese patent application (OPI) Nos. 2437/77 and 107246/81; or a phthalocyanine pigment or azo pigment and an alkali-soluble phenolic resin as disclosed in Japanese patent application (OPI) Nos. 105254/80, 16125/70, 150953/83, and 162961/83.
However, since the above-mentioned plate making process involves removal of the photosensitive layer corresponding to the non-image areas by dissolution, it requires a large-size apparatus and is too time-consuming to achieve a reasonable production rate. Also, the use of an organic solvent as the processing solution (such as ethylene glycol, glycerin, methanol, ethanol, etc.), involves problems of cost, safety, environmental pollution, working hygiene, and the like.
It has been proposed to provide a specific resin layer capable of being rendered hydrophilic on a usual electrophotographic photoreceptor as described in Japanese Patent Publication No. 5606/70. According to this system, a surface layer comprising a vinyl ether-maleic anhydride copolymer and a hydrophobic resin compatible with the copolymer is provided on the photosensitive layer. After toner image formation, the non-image areas of the surface layer can be rendered hydrophilic by treating with an alkali to hydrolytically opening the acid anhydride ring.
However, since the vinyl ether-maleic anhydride copolymer used in the surface layer becomes water-soluble upon ring-opening, the surface layer is seriously inferior in water resistance even though the vinyl ether-maleic anhydride copolymer is combined with a compatible hydrophobic resin. Therefore, the printing durability of the resulting printing plate was about 500 to 600 prints at most.
It has also been proposed to form a surface layer capable of being rendered hydrophilic, which comprises silylated polyvinyl alcohol as main component and a crosslinking agent as disclosed in Japanese patent application (OPI) No. 217292/86. According to this proposal, after toner image formation, the surface layer can be rendered hydrophilic by hydrolysis of the silylated polyvinyl alcohol on the non-image areas. Further, in order to retain fine strength after becoming hydrophilic, the degree of silylation of polyvinyl alcohol is controlled, and the remaining hydroxyl group is crosslinked by the crosslinking agent.
The above publication describes that the provision of such a surface layer would prevent background stains of prints and increase printing durability. However, it has been shown by evaluation of actual use that the resulting printing plate still does not satisfy the stain resistance requirement. Further, due to the nature of the high-molecular reaction, it is difficult to stably produce the silylated polyvinyl alcohol having a controlled degree of silylation by silylating polyvinyl alcohol with a silylating agent to a desired degree. In addition, due to the limitations on the chemical structure of the hydrophilic polymer, it is difficult to exclude all adverse influences of the surface layer upon the functions of an electrophotographic photoreceptor, such as charging properties, quality of a reproduced image (e.g., dot reproducibility and resolving power of image areas, resistance to background fog of nonimage areas, etc.), and light sensitivity.